It is already known for such a one-piece device to inlude a degassing passage for degassing the liquid flowing through the heat exchanger, in order to prevent bubbles of air or gas conveyed by the liquid from collecting at various points in the engine cylinder head and thus forming hot spots.
When the water box is disposed generally vertically, bubbles of air or gas conveyed by the engine cooling liquid tend to collect in the top of the water box. For degassing, a tube is provided which is fixed, for example, to the water box and which puts the top end of the water box into communication with the expansion chamber by opening out into the expansion chamber below the level of the liquid which is contained therein in normal operation, in order to avoid air being sucked back into the cooling circuit when the engine is switched off. This tube is inconvenient to install and fix in place, and as a result it increases the most of the heat exchanger.
Preferred embodiments of the present invention avoid this drawback while ensuring that the cooling liquid is still properly degassed.